Method and apparatus for improved vibration isolation

ABSTRACT

A vibration isolator is disclosed having a housing which defines a fluid chamber. A piston, which is movable to and from a down position, is disposed within the housing. A vibration isolation fluid is disposed within the fluid chamber. A passage having a predetermined diameter extends through the piston to permit the vibration isolation fluid to flow from one fluid chamber to the other. An elastic element is provided for reducing transmission of vibrations from the piston to the housing when the piston is at the down position.

TECHNICAL FIELD

The present invention relates in general to vibration control. Thepresent invention relates generally to a method and apparatus forisolating mechanical vibrations in a structure or body which is subjectto harmonic or oscillating displacements or forces, and is of particularutility in the field of aircraft, in particular, helicopters and otherrotary wing aircraft.

DESCRIPTION OF THE PRIOR ART

For many years, effort has been directed toward the design of apparatusfor isolating a vibrating body from transmitting its vibrations toanother body. Such apparatus are useful in a variety of technical fieldsin which it is desirable to isolate the vibration of an oscillating orvibrating device, such as an engine, from the remainder of thestructure. Typical vibration isolation and attenuation devices(“isolators”) employ various combinations of the mechanical systemelements (springs and mass) to adjust the frequency responsecharacteristics of the overall system to achieve acceptable levels ofvibration in the structures of interest in the system. One field inwhich these isolators find a great deal of use is in aircraft, whereinvibration-isolation systems are utilized to isolate the fuselage orother portions of an aircraft from mechanical vibrations, such asharmonic vibrations, which are associated with the propulsion system,and which arise from the engine, transmission, and propellers or rotorsof the aircraft.

A simple force equation for vibration is set forth as follows:F=m{umlaut over (x)}+c{dot over (x)}+kx

Vibration isolators are distinguishable from dampening devices. A truevibration isolator utilizes acceleration of a tuning mass body m{umlautover (x)} to cancel the displacement of vibration kx. On the other hand,a dampening device is concerned with velocity c{dot over (x)} of a fluidor other body which may involve restricting flow, and does not cancelvibration, but merely dissipates it. Unfortunately, dampening devicesare often erroneously referred to as isolators.

One important engineering objective during the design of an aircraftvibration-isolation system is to minimize the length, weight, andoverall size, including cross-section, of the isolation device. This isa primary objective of all engineering efforts relating to aircraft. Itis especially important in the design and manufacture of helicopters andother rotary wing aircraft, such as tilt rotor aircraft, which arerequired to hover against the dead weight of the craft, and which are,thus, somewhat constrained in their payload in comparison withfixed-wing aircraft.

Another important engineering objective during the design ofvibration-isolation systems is the conservation of the engineeringresources that have been expended in the design of other aspects of theaircraft or in the vibration-isolation system. In other words, it is animportant industry objective to make incremental improvements in theperformance of vibration isolation systems which do not require radicalre-engineering or complete redesign of all of the components which arepresent in the existing vibration-isolation systems.

A marked departure in the field of vibration isolation, particularly asapplied to aircraft and helicopters, is disclosed in commonly assignedU.S. Pat. No. 4,236,607, titled “Vibration Suppression System,” issued 2Dec. 1980, to Halwes, et al. (Halwes '607). Halwes '607 is incorporatedherein by reference. Halwes '607 discloses a vibration isolator in whicha dense, low-viscosity fluid is used as the “tuning” mass tocounterbalance, or cancel, oscillating forces transmitted through theisolator. This isolator employs the principle that the acceleration ofan oscillating mass is 180° out of phase with its displacement.

Halwes '607, it was recognized that the inertial characteristics of adense, low-viscosity fluid, combined with a hydraulic advantageresulting from a piston arrangement, could harness the out-of-phaseacceleration to generate counter-balancing forces to attenuate or cancelvibration. Halwes '607 provided a much more compact, reliable, andefficient isolator than was provided in the prior art. The originaldense, low-viscosity fluid contemplated by Halwes '607 was mercury,which is toxic and highly corrosive.

Since Halwes' early invention, much of the effort in this area has beendirected toward replacing mercury as a fluid or to varying the dynamicresponse of a single isolator to attenuate differing vibration modes. Anexample of the latter is found in commonly assigned U.S. Pat. No.5,439,082, titled “Hydraulic Inertial Vibration Isolator,” issued 8 Aug.1995, to McKeown, et al. (McKeown '082). McKeown '082 is incorporatedherein by reference.

Several factors affect the performance and characteristics of theHalwes-type isolator, including the density and viscosity of the fluidemployed, the relative dimensions of components of the isolator, and thelike. One improvement in the design of such isolators is disclosed incommonly assigned U.S. Pat. No. 6,009,983, titled “Method and Apparatusfor Improved Isolation,” issued 4 Jan. 2000, to Stamps et al. (Stamps'983). In Stamps '983, a compound radius at the each end of the tuningpassage was employed to provide a marked improvement in the performanceof the isolator. Stamps '983 is incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. However, the invention itself, as well as,a preferred mode of use, and further objectives and advantages thereof,will best be understood by reference to the following detaileddescription when read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a perspective view of a helicopter according to the presentapplication;

FIG. 2A is a plan view of a tilt rotor aircraft according to the presentapplication in an airplane mode;

FIG. 2B is a perspective view of a tilt rotor aircraft according to thepresent application in a helicopter mode;

FIG. 3 is a perspective view of a quad tilt rotor aircraft according tothe present application in an airplane mode;

FIG. 4A is a cross-sectional view of a prior-art liquid inertiavibration eliminator;

FIG. 4B is a force diagram of the prior-art liquid inertia vibrationeliminator of FIG. 4A;

FIG. 5 is a perspective view of an airframe roof beam and pylon assemblyaccording to the present application;

FIG. 6A is a side view of an embodiment of a liquid inertia vibrationeliminator unit according to the present application;

FIG. 6B is a first cross-sectional view of the liquid inertia vibrationeliminator unit shown in FIG. 6A taken at 6 b-6 b in FIG. 6A;

FIG. 6C is a second cross-sectional view of the liquid inertia vibrationeliminator unit shown in FIG. 6A taken at 6 c-6 c in FIG. 6A;

FIG. 7A is a side view of another embodiment of a liquid inertiavibration eliminator unit according to the present application;

FIG. 7B is a cross-sectional view of the liquid inertia vibrationeliminator unit shown in FIG. 7A taken at 7 b-7 b in FIG. 7A;

FIG. 8A is a an inboard side view of another embodiment of a liquidinertia vibration eliminator unit according to the present disclosure asinstalled on an aircraft;

FIG. 8B is a an outboard side view of the liquid inertia vibrationeliminator unit shown in FIG. 8A;

FIG. 8C is a cross-sectional view of the liquid inertia vibrationeliminator unit shown in FIGS. 8A and 8B taken at 8 b-8 b in FIG. 8A;

FIG. 9A is a plan view of the C-ring shown in FIGS. 8A-8C; and

FIG. 9B is a cross-sectional view of the C-ring shown in FIG. 9A takenat 9 b-9 b in FIG. 9A.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 in the drawings, a helicopter 11 is illustrated.Helicopter 11 has a fuselage 13 and a main rotor assembly 15, includingmain rotor blades 17 and a main rotor shaft 18. Helicopter 11 has a tailrotor assembly 19, including tail rotor blades 21 and a tail rotor shaft20. Main rotor blades 17 generally rotate about a longitudinal axis 16of main rotor shaft 18. Tail rotor blades 21 generally rotate about alongitudinal axis 22 of tail rotor shaft 20. Helicopter 11 also includesa vibration isolation system according to the present disclosure forisolating fuselage 13 or other portions of helicopter 11 from mechanicalvibrations, such as harmonic vibrations, which are associated with thepropulsion system and which arise from the engine, transmission, androtors of helicopter 11.

The present disclosure applies also to other types of rotary wingaircraft. Referring now to FIGS. 2A and 2B in the drawings, a tilt rotoraircraft 111 is illustrated. As is conventional with tilt rotoraircraft, rotor assemblies 113 a and 113 b are carried by wings 115 aand 115 b, and are disposed at end portions 116 a and 116 b of wings 115a and 115 b, respectively. Tilt rotor assemblies 113 a and 113 b includenacelles 120 a and 120 b, which carry the engines and transmissions oftilt rotor aircraft 111, as well as rotor hubs 119 a and 119 b onforward ends 121 a and 121 b of tilt rotor assemblies 113 a and 113 b,respectively.

Tilt rotor assemblies 113 a and 113 b move or rotate relative to wingmembers 115 a and 115 b between a helicopter mode, in which tilt rotorassemblies 113 a and 113 b are tilted upward such that tilt rotoraircraft 111 flies like a conventional helicopter, and an airplane mode,in which tilt rotor assemblies 113 a and 113 b are tilted forward suchthat tilt rotor aircraft 111 flies like a conventional propeller-drivenaircraft. In FIG. 2A, tilt rotor aircraft 111 is shown in the airplanemode; in FIG. 2B, tilt rotor aircraft 111 is shown in the helicoptermode. As shown in FIGS. 2A and 2B, wings 115 a and 115 b are coupled toa fuselage 114. Tilt rotor aircraft 111 also includes a vibrationisolation system according to the present disclosure for isolatingfuselage 114 or other portions of tilt rotor aircraft 111 frommechanical vibrations, such as harmonic vibrations, which are associatedwith the propulsion system and which arise from the engines,transmissions, and rotors of tilt rotor aircraft 111.

Referring now to FIG. 3 in the drawings, a quad tilt rotor aircraft 211is illustrated. As with the tilt rotor aircraft of FIGS. 2A and 2B,rotor assemblies 213 a, 213 b, 213 c, and 213 d are carried by wings 215a, 215 b, 215 c, and 215 d, respectively. Tilt rotor assemblies 213 a,213 b, 213 c, and 213 d include nacelles 220 a, 220 b, 220 c, and 220 d,which carry the engines and transmissions of quad tilt rotor aircraft211, as well as rotor hubs 219 a, 219 b, 219 c, and 219 d on forwardends of tilt rotor assemblies 213 a, 213 b, 213 c, and 213 d,respectively.

Tilt rotor assemblies 213 a, 213 b, 213 c, and 213 d move or rotaterelative to wing members 215 a, 215 b, 215 c, and 215 d between ahelicopter mode, in which tilt rotor assemblies 213 a, 213 b, 213 c, and213 d are tilted upward such that quad tilt rotor aircraft 211 flieslike a conventional helicopter, and an airplane mode, in which tiltrotor assemblies 213 a, 213 b, 213 c, and 213 d are tilted forward suchthat quad tilt rotor aircraft 211 flies like a conventionalpropeller-driven aircraft. In FIG. 3, quad tilt rotor aircraft 211 isshown in the airplane mode. As shown in FIG. 3, wings 215 a, 215 b, 215c, and 215 d are coupled to a fuselage 214. Tilt rotor aircraft 211 alsoincludes a vibration isolation system according to the presentdisclosure for isolating fuselage 214 or other portions of quad tiltrotor aircraft 211 from mechanical vibrations, such as harmonicvibrations, which are associated with the propulsion system and whicharise from the engines, transmissions, and rotors of quad tilt rotoraircraft 211.

It should be understood that the vibration isolation system according tothe present disclosure may be used with any aircraft on which it wouldbe desirable to have vibration isolation, including unmanned aerialvehicles that are remotely piloted.

Referring now to FIG. 4A in the drawings, a prior-art liquid inertiavibration eliminator (LIVE unit) 327 for use on an aircraft isillustrated. Prior-art LIVE unit 327 includes a metal housing 343 thathas a hollow, generally cylindrical interior. A metal piston 347 ofselected cross-sectional diameter is disposed within the interior ofhousing 343. Housing 343 would typically be coupled to the fuselage ofan aircraft (not shown) and piston 347 would typically be coupled to thetransmission and propulsion system of the aircraft (not shown),generally referred to as a pylon assembly, at an attachment bracket 363.In such an arrangement, the fuselage serves as the body to be isolatedfrom vibration, and the transmission of the aircraft serves as thevibrating body. An elastomeric seal and spring member 349 resilientlyseals piston 347 within the interior of housing 343.

A fluid chamber 361 is defined by the interior of housing 343 and piston347 and is sealed against leakage by elastomer member 349. Aknown-density, low-viscosity vibration-isolation fluid, also referred toas tuning fluid, is disposed within fluid chamber 361. In addition tosealing the vibration-isolation fluid in fluid chamber 361, elastomermember 349 functions as a spring to permit piston 347 to move oroscillate relative to housing 343, while maintaining piston 347 in acentral location in housing 343 when no load is applied.

A tuning port or passage 357 extends centrally through piston 347 andpermits the vibration-isolation fluid to move from one end of fluidchamber 361 to the other. A conical flow diverter 351 is provided ateach end of housing 343 and is aligned with and generally opposes theopening at each end of tuning passage 357. Each conical flow diverter351 enhances fluid flow by decelerating the vibration-isolation fluid asit flows from each end of the fluid chamber into and out of passage 357.

Referring now to FIG. 4B in the drawings, a mechanical equivalent model375 for the prior art LIVE unit 327 of FIG. 4A is illustrated. Inmechanical equivalent model 375, a box 377 represents the mass of thefuselage M_(fuselage); a box 379 represents the mass of the pylonassembly M_(pylon), and a box 381 represents the mass of the tuning massM_(t), in this case, the vibration-isolation fluid. A vibratory forceF·sin(ωt) is generated by the engine, transmission, and propulsionsystem. Force F·sin(ωt) is a function of the frequency of vibration ofthe transmission and propulsion system.

Force F·sin(ωt) causes an oscillatory displacement u_(p) of the pylonassembly; an oscillatory displacement of the fuselage u_(f); and anoscillatory displacement of the tuning mass u_(t). Elastomer member 349is represented by a spring 382 disposed between the fuselageM_(fuselage) and the pylon assembly M_(pylon). Spring 382 has a springconstant k.

In mechanical equivalent model 375, tuning mass M_(t) functions as ifcantilevered from a first fulcrum 383 attached to the pylon assemblyM_(pylon), and a second fulcrum 385 attached to the fuselageM_(fuselage). The distance a from first fulcrum 383 to second fulcrum385 represents the cross-sectional area of tuning port 357, and thedistance b from first fulcrum 383 to the tuning mass M_(t) representsthe cross-sectional area of piston 347, such that an area ratio, orhydraulic ratio, R is equal to the ratio of b to a.

Referring again to FIG. 4A, the metal housing 343 includes a pluralityof metal downstops 390. In some situations, such as when the aircraft isresting on the ground, the metal piston 347 is in contact with the metaldownstops 390. This metal to metal contact can allow for undesirabletransfer of vibrations from the engine, transmission, and propulsionsystem, through the piston 347, to the fuselage via the downstops 390and housing 343. On the other hand, the present disclosure provides ameans of reducing vibration levels in the fuselage that originate fromthe engine, transmission, and propulsion system.

Referring now to FIG. 5 in the drawings, a perspective view of anairframe roof beam and pylon assembly 400 is illustrated. The assembly400 includes a partial airframe roof beam 402 and pylon support beams404 a and 404 b mounted to the airframe 402 for supporting the pylon.The pylon generally refers to the propulsion system, which includes arotor assembly (not shown) and a transmission 406. A main rotor shaft408, on which a main rotor assembly (not shown) can be mounted, extendsupwardly from the transmission 406. The transmission 406 can be drivenby an engine (not shown) via a shaft 410 to turn the main rotor shaft408. The resulting vibrations are isolated from the airframe 402 by LIVEunits 412 a and 412 b.

Referring now to FIGS. 6A-6C in the drawings, an embodiment of a LIVEunit 500 is shown that can be used as the LIVE units 412 a and 412 b.FIG. 6A shows a side view of the LIVE unit 500. The LIVE unit 500 isparticularly well-suited for use on a helicopter, but can also be usedon other aircraft. FIG. 6B shows a cross-sectional view of the LIVE unit500 taken along section line 6 b-6 b in FIG. 6A. FIG. 6C shows across-sectional view of the LIVE unit 500 taken along section line 6 c-6c in FIG. 6A.

Referring to FIG. 6B, the LIVE unit 500 includes a housing 502 having ahollow, generally cylindrical interior. Housing 502 would typically becoupled to the fuselage of the aircraft, i.e., the body being isolatedfrom the vibration. A piston 504 of selected cross-sectional diameter isdisposed within the interior of housing 502. Piston 504 would typicallybe coupled to the pylon of the aircraft, i.e., the rotor assembly andtransmission, which are a primary source of vibrations. An elastomericseal and spring member 506 resiliently seals piston 504 within theinterior of housing 502.

A fluid chamber 508 is defined by the interior of housing 502 and piston504. A known-density, vibration-isolation fluid, also referred to astuning fluid, is disposed within fluid chamber 508. Tuning fluid ispreferably organic with non-corrosive properties having low-viscosityand high density, similar to the SPF I family of fluids available fromLord Corporation. In addition to sealing tuning fluid within fluidchamber 508, elastomeric member 506 functions as a spring to permitpiston 504 to move or oscillate relative to housing 502, whilemaintaining piston 504 in a central location within housing 502 when noload is applied.

A tuning port 512 extends centrally through piston 504 and permitstuning fluid to move from one end of fluid chamber 508 to the other. Aconical flow diverter 514 is provided at each end of housing 502 and isaligned with and generally opposes the opening at each end of tuningpassage 512. Each conical flow diverter 514 enhances fluid flow bydecelerating the vibration-isolation fluid as it flows from each end ofthe fluid chamber into and out of passage 512.

Referring next to both FIGS. 6B and 6C, the housing 502 includes aplurality of downstops 516, including downstops 516 a, 516 b, 516 c, and516 d. Elastic members 518 a-d, for example formed of rubber, extendbeneath the downstops 516 a-d, respectively, such that elastic members518 a-d are provided between the downstops 516 a-d and the housing 502.The piston 504 is movable relative to the housing 502 in the directionsindicated by arrows A and B. From the position illustrated, the pistoncan move in the direction indicated by arrow A (down as shown in FIG.6B) to a down position, where the travel of the piston is stopped by thedownstops 516 a-d, and from the down position in the direction indicatedby arrow B (up as shown in FIG. 6B). The placement of the elasticmembers 518 a-d between the downstops 516 a-d and the housing 502provides for a compliant base, rather than a hard base, for the pylon torest on when the piston 504 is in the down position, for example whenthe aircraft is on the ground. If the pylon rests on a hard surface, thepylon lateral mode natural frequency increases and approaches main rotor4/rev harmonic frequency (i.e., blade passage frequency). This resonantor near-resonant condition results in increased vibration levels, whichare transferred to the fuselage through the downstop and housing. Byincorporating the elastic members 518 a-d, the pylon lateral modenatural frequency does not approach resonance with 4/rev; therefore, thevibration levels do not significantly increase when the pylon rests onthe downstops 516 a-d.

Referring next to FIGS. 7A and 7B, an alternative embodiment is shown asLIVE unit 600, which can also be used as the LIVE units 412 a and 412 b(FIG. 5). FIG. 7A shows a side view of the LIVE unit 600. The LIVE unit600 is particularly well-suited for use on a helicopter, but can also beused on other aircraft. FIG. 7B shows a cross-sectional view of the LIVEunit 600 taken along section line 7 b-7 b in FIG. 7A.

Referring to FIG. 7B, the LIVE unit 600 includes a housing 602 having ahollow, generally cylindrical interior. Housing 602 would typically becoupled to the fuselage of the aircraft, i.e., the body being isolatedfrom the vibration. A piston 604 of selected cross-sectional diameter isdisposed within the interior of housing 602. Piston 604 would typicallybe coupled to the pylon of the aircraft, i.e., the rotor assembly andtransmission, which are a primary source of vibrations. An elastomericseal and spring member 606 resiliently seals piston 604 within theinterior of housing 602.

A fluid chamber 608 is defined by the interior of housing 602 and piston604. A known-density, vibration-isolation fluid, also referred to astuning fluid, is disposed within fluid chamber 608. Tuning fluid ispreferably organic with non-corrosive properties having low-viscosityand high density, similar to the SPF I family of fluids available fromLord Corporation. In addition to sealing tuning fluid within fluidchamber 608, elastomeric member 606 functions as a spring to permitpiston 604 to move or oscillate relative to housing 602, whilemaintaining piston 604 in a central location within housing 602 when noload is applied.

A tuning port 612 extends centrally through piston 604 and permitstuning fluid to move from one end of fluid chamber 608 to the other. Aconical flow diverter 614 is provided at each end of housing 602 and isaligned with and generally opposes the opening at each end of tuningpassage 612. Each conical flow diverter 614 enhances fluid flow bydecelerating the vibration-isolation fluid as it flows from each end ofthe fluid chamber into and out of passage 612.

The housing 602 includes a plurality of downstops 616, includingdownstops 616 a and 616 b. The housing 602 can include an array ofdownstops, for example arranged in the same or similar manner as thedownstops 516 a-d are arranged in FIG. 6C. Elastic member 618, forexample formed of rubber, extends beneath the piston 604 from theelastomeric member 606 towards the passage 612. The elastic member 618extends over at least enough of the piston 604 so that the elasticmember 618 is between the downstops 616 a and 616 b and the piston 604.The piston 604 is movable relative to the housing 602 in the directionsindicated by arrows A and B. From the position illustrated, the pistoncan move in the direction indicated by arrow A (down as shown in FIG.7B) to a down position, where the travel of the piston is stopped by thedownstops 616 a and 616 b, and from the down position in the directionindicated by arrow B (up as shown in FIG. 7B). The placement of theelastic member 618 between the downstops 616 a and 616 b and the piston604 provides for a compliant base, rather than a hard base, for thepylon to rest on when the piston 604 is in the down position, forexample when the aircraft is on the ground. If the pylon rests on a hardsurface, the pylon lateral mode natural frequency increases andapproaches main rotor 4/rev harmonic frequency (i.e., blade passagefrequency). This resonant or near-resonant condition results inincreased vibration levels, which are transferred to the fuselagethrough the downstop and housing. By incorporating the elastic member618, the pylon lateral mode natural frequency does not approachresonance with 4/rev; therefore, the vibration levels do notsignificantly increase when the pylon rests on the downstops 616 a and616 b.

Referring next to FIGS. 8A-8C, another alternative embodiment is shownas LIVE unit 700, which can also be used as the LIVE units 412 a and 412b (FIG. 5). The LIVE unit 700 is particularly well-suited for use on ahelicopter, but can also be used on other aircraft. FIG. 8A shows aninboard side view of the LIVE unit 700 as installed on an aircraft; FIG.8B shows an outboard side view of the LIVE unit 700 as installed on anaircraft. The LIVE unit 700 is connected to an airframe (not shown) ofan aircraft via a pylon support beam 800 similar to 404 a and 404 b(FIG. 5). The LIVE unit 700 is also connected to a pylon assembly (notshown) via a transmission attachment cap 802. A portion of the cap 802is not shown in FIG. 8B in order that the LIVE unit 700 may be moreclearly shown. In this embodiment, a C-ring assembly 804 is providedaround at least a portion of the LIVE unit 700 between the transmissionattachment cap 802 and the pylon support beam 800 similar to 404 a and404 b (FIG. 5).

FIG. 8C shows a cross-sectional view of the LIVE unit 700. The LIVE unit700 includes a housing 702 having a hollow, generally cylindricalinterior. Housing 702 would typically be coupled to the fuselage of theaircraft, i.e., the body being isolated from the vibration. A piston 704of selected cross-sectional diameter is disposed within the interior ofhousing 702. Piston 704 would typically be coupled to the pylon of theaircraft, i.e., the engine and transmission, which are a primary sourceof vibrations. An elastomeric seal and spring member 706 resilientlyseals piston 704 within the interior of housing 702.

A fluid chamber 708 is defined by the interior of housing 702 and piston704. A known-density, vibration-isolation fluid, also referred to astuning fluid, is disposed within fluid chamber 708. Tuning fluid ispreferably organic with non-corrosive properties having low-viscosityand high density, similar to the SPF I family of fluids available fromLord Corporation. In addition to sealing tuning fluid within fluidchamber 708, elastomeric member 706 functions as a spring to permitpiston 704 to move or oscillate relative to housing 702, whilemaintaining piston 704 in a central location within housing 702 when noload is applied.

A tuning port 712 extends centrally through piston 704 and permitstuning fluid to move from one end of fluid chamber 708 to the other. Aconical flow diverter 714 is provided at each end of housing 702 and isaligned with and generally opposes the opening at each end of tuningpassage 712. Each conical flow diverter 714 enhances fluid flow bydecelerating the vibration-isolation fluid as it flows from each end ofthe fluid chamber into and out of passage 712.

The housing 702 includes a plurality of downstops 716, includingdownstops 716 a and 716 b. The housing 702 can include an array ofdownstops, for example arranged in the same or similar manner as thedownstops 516 a-d are arranged in FIG. 6C.

The C-ring assembly 804 is provided around at least a portion of theLIVE unit 700 between the transmission attachment cap 802 and the pylonsupport beam 800 similar to 404 a and 404 b (FIG. 5).

Referring to FIG. 9A, a top view of an embodiment of the C-ring assembly804 is illustrated. FIG. 9B shows a cross-sectional view of the C-ringassembly taken along section line 9 b-9 b shown in FIG. 9A. The C-ringassembly includes a rigid base 806, preferably formed of metal or otherrigid material. Elastic layers 808 a and 808 b, for example formed ofrubber, each extend over respective portions of the upper surface of therigid base 806. The placement of the elastic layers 808 a and 808 b ischosen so that, when the C-ring assembly 804 is properly installed, theelastic layers will be positioned between the piston 704, or otherelements rigidly connected to the piston 704 such as the cap 802, andthe housing 702, or other elements rigidly connected to the housing 702such as the support bracket 800. In alternative embodiments, a singlecontinuous elastic layer can be used in place of separate layers 808 aand 808 b so long as the single elastic layer can still be positionedbetween the piston 704, or other elements rigidly connected to thepiston 704 such as the cap 802, and the housing 702, or other elementsrigidly connected to the housing 702 such as the support bracket 800.

Referring again to FIG. 8C, the piston 704 is movable relative to thehousing 702 in the directions indicated by arrows A and B. From theposition illustrated, the piston can move in the direction indicated byarrow A (down as shown in FIG. 8C) to a down position, where the travelof the piston is stopped by the C-ring 804, and from the down positionin the direction indicated by arrow B (up as shown in FIG. 8C). Theplacement of the elastic layers 808 a and 808 b between the piston 704,or other elements rigidly connected to the piston 704 such as the cap802, and the housing 702, or other elements rigidly connected to thehousing 702 such as the support bracket 800, provides for a compliantbase, rather than a hard base, for the pylon to rest on when theaircraft is on the ground. If the pylon rests on a hard surface, thepylon lateral mode natural frequency increases and approaches main rotor4/rev harmonic frequency (i.e., blade passage frequency). This resonantor near-resonant condition results in increased vibration levels, whichare transferred to the fuselage through the downstop and housing. Byincorporating the elastic members 808 a and 808 b, the pylon lateralmode natural frequency does not approach resonance with 4/rev;therefore, the vibration levels do not significantly increase when thepylon rests on the C-ring assembly 804 having the elastic layers 808 aand 808 b.

It is apparent that an invention with significant advantages has beendescribed and illustrated. Although the present invention is shown in alimited number of forms, it is not limited to just these forms, but isamenable to various changes and modifications without departing from thespirit thereof.

The invention claimed is:
 1. A vibration isolator, comprising: a housingdefining a fluid chamber; a fluid disposed within the fluid chamber; apiston resiliently disposed within the housing and movable to and from adown position, the housing having one or more downstops; a means forresiliently coupling the piston to the housing; and an elastic elementfor reducing transmission of vibrations from the piston to the housingwhen the piston is at the down position, wherein the elastic elementprovides a compliant base for the piston to rest on when the piston isin the down position; wherein the elastic element is spaced apart fromat least one of the piston and the downstop.
 2. The vibration isolatoraccording to claim 1, further comprising at least one downstoppositioned so as to prevent the piston from traveling beyond the downposition.
 3. The vibration isolator according to claim 2, wherein theelastic element is positioned between the downstop and the housing. 4.The vibration isolator according to claim 2, wherein the elastic elementis positioned between the piston and the downstop.
 5. The vibrationisolator according to claim 4, wherein the elastic element is connectedto the piston.
 6. The vibration isolator according to claim 5, whereinthe elastic element is connected to the means for resiliently couplingthe piston to the housing.
 7. The vibration isolator according to claim1, further comprising a C-ring that at least partially extends aroundthe piston, wherein the elastic element is a layer of the C-ring.
 8. Thevibration isolator according to claim 7, wherein the C-ring furthercomprises a rigid base layer.
 9. An aircraft, comprising: an airframe; apropulsion system carried by the airframe; and a vibration isolatordisposed between the airframe and the propulsion system, the vibrationisolator comprising: a housing defining a fluid chamber; a fluiddisposed within the fluid chamber; a piston resiliently disposed withinthe housing and movable to and from a down position, the housing havingone or more downstops; a means for resiliently coupling the piston tothe housing; and an elastic element for reducing transmission ofvibrations from the piston to the housing when the piston is at the downposition, wherein the elastic element provides a compliant base for thepiston to rest on when the piston is in the down position; wherein theelastic element is spaced apart from at least one of the piston and thedownstop; whereby oscillatory forces generated by the propulsion systemare reduced.
 10. The aircraft according to claim 9, further comprisingat least one downstop positioned so as to prevent the piston fromtraveling beyond the down position.
 11. The aircraft according to claim10, wherein the elastic element is positioned between the downstop andthe housing.
 12. The aircraft according to claim 10, wherein the elasticelement is positioned between the piston and the downstop.
 13. Theaircraft according to claim 12, wherein the elastic element is connectedto the piston.
 14. The aircraft according to claim 13, wherein theelastic element is connected to the means for resiliently coupling thepiston to the housing.
 15. The aircraft according to claim 9, furthercomprising a C-ring that at least partially extends around the piston,wherein the elastic element is a layer of the C-ring.
 16. The aircraftaccording to claim 15, wherein the C-ring further comprises a rigid baselayer.
 17. A method of vibration isolation, comprising: providing ahousing defining a fluid chamber; providing a fluid disposed within thefluid chamber; providing a piston resiliently disposed within thehousing and movable to and from a down position, the housing having oneor more downstops; providing a means for resiliently coupling the pistonto the housing; and reducing transmission of vibrations from the pistonto the housing when the piston is at the down position via an elasticelement, the elastic element providing a compliant base for the pistonto rest on when the piston is in the down position; wherein the elasticelement is spaced apart from at least one of the piston and thedownstop.
 18. The method according to claim 17, further comprising:providing at least one downstop positioned so as to prevent the pistonfrom traveling beyond the down position; and providing the elasticelement between the downstop and the housing.
 19. The method accordingto claim 17, further comprising: providing at least one downstoppositioned so as to prevent the piston from traveling beyond the downposition; and providing the elastic element between the piston and thedownstop.
 20. The method according to claim 17, further comprising:providing a C-ring that at least partially extends around the piston;and providing the elastic element as a layer of the C-ring.